The present invention generally relates to balancing stacked components of rotating machinery, and more specifically, to achieving group balance of a turbine rotor assembly.
Gas turbine engines include rotating components such as fans, compressors, and turbines. The components are clamped together axially either by a tieshaft or bolted flange joints. In many applications, nuts and bolts are used to apply compressive forces on multiple components, securing them in a stacked relation on the shaft. The compressive force through the components is equal to the tensile force in the shaft, which stretches proportionally to the original shaft length.
In gas turbine engines, a nut is often used on the end of a threaded shaft to secure and position the engine components relative to the shaft. The shaft traditionally has a radial flange extending outward at one end to provide an abutting surface and threads for the nut at the opposite end. The engine components are stacked along the shaft such that the shaft extends through the center of the components. The nut is threaded to the shaft to apply a compressive force through the components that secures them in place relative to the shaft, and thus, engages pilots of the components. Proper balancing and piloting of the components on the shaft is required to achieve an acceptable balance of the group when assembled. The tie-shaft may serve other functions in addition to securing the outer stack of components, such as providing a location for mounting of bearings, and power transfer to another shaft via a spline. Alternatively, a single shaft and nut system may serve simply to axially secure an outer stack of rotating components.
The process of balancing a rotor group, e.g., for a gas turbine engine component stack, can be time consuming and costly. The primary sources of unbalance in a rotor group are component unbalance and rotor bow. A problem occurs when the stacked components are axially loaded, e.g., with a nut threaded on a tie-shaft. Non-parallel features of the components cause rotor bow resulting in unbalance of the rotor group. Component unbalance is typically very low; often less than 50% of the desired group unbalance level. Rotor bow can result in components having an unbalance level when assembled in the group level much larger than the level they were balanced to as a component. Typical increases in component unbalance due to rotor bow can be in the order of 2-5 times (2× to 5×).
Each of the rotor components may be balanced before assembly of the rotor group. The balance of the group is then checked after assembly. If the group does not meet its established limits, a component of the group must be rotated. Balance is again checked and, if necessary, another component is rotated. This process is repeated in an iterative fashion until group balance is achieved. Clocking of components can be time consuming, leading to higher product cost. Clocking of a single component can take 30 minutes or more. In many situations, components are pressed onto other components, resulting in even more time to clock the components. Many groups can require clocking of components four or five time
Various designs controlling rotor runouts in relation to the associated static structure have been proposed in the prior art. One such conventional design is disclosed in U.S. Pat. No. 4,901,523 to Huelster (“Huelster patent”). The Huelster patent discloses an adjustable annular shim pack that is used to minimize running clearances between compressor/turbine blade tips and a static structure. The design disclosed in the Huelster patent is not capable of controlling group balance in the case presented by Huelster. By using the shim pack of Huelster, correcting for running clearances might increase rotor unbalance.
As can be seen, there is a need for improved apparatus and methods for achieving group balance of stacked components, including balance repeatability.